Field of the Invention
The present invention relates to honeycomb structures. More particularly the present invention relates to a honeycomb structure with low heat capacity and high thermal diffusivity and that follows the ambient temperature change rapidly.
Description of the Related Art
Exhaust gas emitted from an internal combustion engine, such as a diesel engine and a gasoline engine, or various types of combustion devices contain a lot of particulate substances (hereinafter called “particulate matter” or “PM” as well) mainly containing soot. If this PM is discharged directly to the air, environment pollution will be caused, and so an exhaust system for exhaust gas includes a particulate filter to trap PM. For instance, examples of a particulate filter used to purify exhaust gas emitted from a diesel engine or a gasoline engine include a diesel particulate filter (DPF) or a gasoline particulate filter (GPF). Such a DPF or a GPF includes a honeycomb structure, for example, having a porous partition wall defining a plurality of cells serving as a through channel of exhaust gas.
The exhaust gas as stated above contains harmful substances, such as NOx, CO and HC as well. In order to reduce the amount of harmful substances in exhaust gas to purify the exhaust gas, catalyst reactions are widely used. For the purification of exhaust gas using such a catalyst reaction, a honeycomb structure is used as a catalyst carrier to load a catalyst.
As the honeycomb structure, there is proposed, for example, a honeycomb structure formed by a porous sintered body which contains silicon as a main component where silicon components are covalently bonded, and which has the porosity of 20 to 70% (see Patent Document 1, for example). Moreover, as the honeycomb structure, there is proposed another honeycomb structure which includes refractory particles as aggregate and metallic silicon, and which is porous and has the porosity in the range of 30 to 90% (see Patent Document 2, for example). The honeycomb structure described in Patent Document 2 has a structure in which the refractory particles are bonded together by means of metallic silicon partially at the surface of the refractory particles, and the content of the metallic silicon is within the range of 10 to 45% by weight with respect to the total amount of the refractory particle and the metallic silicon.
Another honeycomb structure functioning as a heater as well as the catalyst carrier also is proposed (see Patent Document 3, for example). For instance, the honeycomb structure described in Patent Document 3 includes a honeycomb structure body having a partition wall and a circumferential wall, and a pair of electrode members disposed laterally on the honeycomb structure body. This honeycomb structure is configured so that, when electricity is applied to the pair of electrode members, the honeycomb structure body produces heat. In the honeycomb structure described in Patent Document 3, the partition wall and the circumferential wall contain silicon carbide particles as the aggregate and silicon as the bonding agent to bond the silicon carbide particles. Then, in order to exert appropriate heat-production characteristics, the honeycomb structure body has volume resistivity at 400° C. that is 1 to 40 Ωcm, and the electrode members have volume resistivity at 400° C. that is 40% or less of the volume resistivity of the honeycomb structure body at 400° C.
Herein since pressure loss increases gradually over time in the honeycomb filters, such as DPF, due to PM deposited inside of the filter, regeneration of such a filter in which PM deposited inside of the honeycomb filter is combusted for removal is often performed in a periodical interval. For instance, as a method for regeneration of a DPF, a regeneration method is known, including raising the temperature of exhaust gas emitted from an engine, and heating the DPF using the high-temperature exhaust gas. As a method for raising the temperature of exhaust gas, an example of the method includes post-injection to temporarily inject fuel excessively at the latter half of the explosion stroke or at the exhaust stroke so as to combust the excessive fuel and so raise the temperature of the exhaust gas. Then, when a honeycomb filter is regenerated by the regeneration method as stated above, it is desirable that the honeycomb filter follow rapidly the ambient temperature change in terms of improvement of fuel efficiency. In order to allow the honeycomb filter to follow the ambient temperature change rapidly, a method of controlling the heat capacity or the thermal diffusivity of the honeycomb filter can be considered, for example.    [Patent Document 1] JP-A-2014-193782    [Patent Document 2] JP-B-4136319    [Patent Document 3] WO 2011/043434